This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-150058, filed May 22, 2000, the entire contents of which are incorporated herein by reference.
The present invention relates to a planar acoustic converting apparatus.
FIG. 1 is a sectional view schematically showing a conventional planar acoustic converting apparatus. The planar acoustic converting apparatus shown in FIG. 1 is disclosed in WO/099/03304 and has a flat yoke 10 formed from a ferromagnetic metal plate such as an iron plate, and permanent magnets 12 attached to one surface of the yoke 10 with their magnetic axes set perpendicular to the surface of the yoke 10. The permanent magnets 12 are arrayed on one major surface of the yoke 10 while being spaced apart from each other by a predetermined gap, and attached to the yoke 10 such that adjacent permanent magnets have opposite polarities.
The planar acoustic converting apparatus shown in FIG. 1 also has a diaphragm 14. This diaphragm 14 is held while being apart from the pole-faces of the permanent magnets 12 by a predetermined distance. The diaphragm 14 has a structure in which spiral coils 18 are formed on both surfaces (or one surface) of an insulating base film 16 in correspondence with the permanent magnets 12. The spiral coils 18 are formed such that each coil 18 surrounds a region being opposed to the magnetic pole of a corresponding permanent magnet 12 and such that, near the boundary between each two coils 18 adjacent to each other, a direction of current-flow through the conductor of one coil 18 is the same as that of another coil 18.
FIG. 2 is a view schematically showing the wiring pattern of the spiral coils 18 shown in FIG. 1. Referring to FIG. 2, reference numeral 18n1 denotes a coil formed on the upper surface of the base film 16, and reference numeral 18n2 denotes a coil formed on the lower surface of the base film in correspondence with the coil 18n1. The coil 18n1 on the upper surface spirals clockwise from the outer to the inner side. On the other hand, the coil 18n2 on the lower surface spirals clockwise from the inner to the outer side. The internal end of the coil 18n1 and that of the coil 18n2 corresponding to the coil 18n1 are electrically connected to each other via a through hole or through stud extending through the base film 16. Hence, the coils 18n1 and 18n2 constitute one coil 18 which spirals clockwise.
Referring to FIG. 2, reference numeral 18m1 denotes a coil formed on the upper surface of the base film 16 to be adjacent to the coil 18n1, and reference numeral 18m2 denotes a coil formed on the lower surface of the base film 16 to be adjacent to the coil 18n2. The coil 18m2 on the lower surface has an outer end connected to that of the adjacent coil 18n2 and spirals counterclockwise from the outer to the inner side. On the other hand, the coil 18m1 on the upper surface spirals counterclockwise from the inner to the outer side. The internal end of the coil 18m1 and that of the coil 18m2 corresponding to the coil 18m1 are electrically connected to each other via a through hole or through stud extending through the base film 16. Hence, the coils 18m1 and 18m2 constitute one coil 18 which spirals counterclockwise.
When the plurality of spiral coils 18 are formed in this way, near the boundary between adjacent coils 18, a current flows through the wire of one coil 18 in the same direction as that of the current flowing through the wire of the other coil 18. Each coil 18 is placed in a magnetic field formed by a corresponding permanent magnet 12 that has a polarity opposite to that of an adjacent permanent magnet 12, as shown in FIG. 1. For this reason, when a current flows in the above way, the diaphragm 14 receives an electromagnetic force by the Fleming""s left-hand rule. That is, as shown in FIG. 2, when magnetic poles N and S of the permanent magnets 12 form magnetic fields H, and currents flow through the coils 18 in the directions of arrows, a force is generated in a direction F. With this principle, the diaphragm 14 vibrates in correspondence with the sound currents flowing through the coils 18.
A planar acoustic converting apparatus of such type can be made as thin as about 5 to 15 mm and can be suitably used for a wall-type TV or notebook personal computer. Such a planar acoustic converting apparatus can also be built in a pillar or sun visor of a car.
However, in a planar acoustic converting apparatus of this type, each coil generates Joule heat. In addition, since the area occupied by the spiral coils 18 on the base film 16 is very large, the influence of heat on the base film 16 cannot be neglected. To prevent this, it has been proposed to use a polyimide film with high heat resistance as the base film 16. However, tan xcex4, which is an index of acoustic absorptivity, of a polyimide film is as low as 0.02, so noise, so-called chattering noise, is readily generated when the diaphragm 14 vibrates. In addition, since a polyimide film is hygroscopic, when a polyimide film is used as the base film 16, the sound quality is expected to change due to a slight extension upon absorbing moisture.
Use of a PET (polyethylene terephthalate) film as the base film 16 has also been proposed. However, a PET film has also poor acoustic absorptivity tan xcex4=0.014, and noise is readily generated when the diaphragm 14 vibrates.
In a planar acoustic converting apparatus of the above type, when the diaphragm 14 largely vibrates, it may hit the permanent magnet 12 to generate impact noise. This problem becomes more conspicuous when the diaphragm 14 slacks due to the above-described heat generation by the coils 18. As a known means for preventing this problem, a flexible material such as polyurethane foam or glass wool is inserted between the diaphragm 14 and the permanent magnets 12. However, such a flexible material hinders the free vibration of the diaphragm 14 to degrade the sound quality.
When the coils 18 receive an electromagnetic force, the diaphragm 14 vigorously vibrates in the direction of thickness. If the adhesive force between the base film 16 and the coils 18m1, 18m2, 18n1, and 18n2 is not sufficiently strong, the coils 18m1, 18m2, 18n1, and 18n2 may peel off from the base film 16. The diaphragm 14 having the plurality of spiral coils 18 formed on one or both surfaces of the base film 16 can be manufactured by the normal flexible printed circuit board manufacturing technology. To effectively prevent the coils 18m1, 18m2, 18n1, and 18n2 from peeling off in such a manufacturing technology, the surfaces of the base film 16 are roughened to increase the adhesive force per unit area, or the conductor width of the coils 18m1, 18m2, 18n1, and 18n2 is increased. However, the former technique can hardly be applied when a thin base film 16 is used to improve the vibration characteristic, and the latter technique is not preferable because the planar acoustic converting apparatus becomes bulky.
It is an object of the present invention to provide a planar acoustic converting apparatus in which generation of noise is suppressed.
It is another object of the present invention to provide a planar acoustic converting apparatus in which impact noise generated by collision of the diaphragm to the permanent magnets can be suppressed without hindering free vibration of the diaphragm.
It is still another object of the present invention to provide a reliable planar acoustic converting apparatus in which the spiral coils of the diaphragm hardly peel off from the base film.
According to the first aspect of the present invention, there is provided a planar acoustic converting apparatus comprising a support having a flat plate portion, a diaphragm comprising an insulating base film having a liquid crystalline polymer film and being opposed to the flat plate portion of the support, and at least one spiral coil provided on one major surface or both major surfaces of the insulating base film, at least one permanent magnet supported by the support and opposing a magnetic pole thereof to the diaphragm, and a holding portion provided to the support and holding the diaphragm such that the diaphragm can vibrate and is positioned apart from the at least one permanent magnet.
According to the second aspect of the present invention, there is provided a planar acoustic converting apparatus comprising a support having a flat plate portion, a diaphragm comprising an insulating base film and being opposed to the flat plate portion of the support, and at least one spiral coil provided on one major surface or both major surfaces of the insulating base film, at least one permanent magnet supported by the support and opposing a magnetic pole thereof to the diaphragm, a damper sheet provided on a surface of the at least one permanent magnet being opposed to the insulating base film, and a holding portion provided to the support and holding the diaphragm such that the diaphragm can vibrate and is positioned apart from the at least one permanent magnet.
According to the third aspect of the present invention, there is provided a planar acoustic converting apparatus comprising a support having a flat plate portion, a diaphragm comprising an insulating base film having a liquid crystalline polymer film and being opposed to the flat plate portion of the support, at least one spiral coil provided on one major surface or both major surfaces of the insulating base film, and an insulation film which covers the at least one spiral coil and the insulating base film, at least one permanent magnet supported by the support and opposing a magnetic pole thereof to the diaphragm, and a holding portion provided to the support and holding the diaphragm such that the diaphragm can vibrate and is positioned apart from the at least one permanent magnet.
The term xe2x80x9cliquid crystalline polymerxe2x80x9d is used with the same meaning and scope as in normal use. That is, the term xe2x80x9cliquid crystalline polymerxe2x80x9d used here includes a polymer that exhibits fluidity and characteristics of a crystal in molten state. Hence, the term xe2x80x9cliquid crystalline polymer filmxe2x80x9d includes a film constituted by such a xe2x80x9cliquid crystalline polymerxe2x80x9d.
The term xe2x80x9ctan xcex4xe2x80x9d represents the degree of conversion of a mechanical energy applied to a film into a thermal energy, i.e., the degree of internal loss, and is used as an index related to the acoustic absorptivity of the film. Letting Exe2x80x2 be the storage elastic modulus, and Exe2x80x3 be the loss elastic modulus, xe2x80x9ctan xcex4xe2x80x9d can be calculated by using the following equation.
tan xcex4=Exe2x80x3/Exe2x80x2
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.